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Animalia
Animals are organisms that form the Animalia. With few exceptions, animals , , are , can , and grow from a hollow sphere of cells, the , during . Over 1.5 million animal have been —of which around 1 million are —but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to . They have with each other and their environments, forming intricate s. The kingdom Animalia includes s, but in colloquial use the term animal often refers only to non-human animals. The study of non-human animals is known as . Most living animal species are in the , a whose members have a body plan. The Bilateria include the —in which many groups of s are found, such as s, s, and s—and the , containing both the s as well as the s, the latter containing the s. Life forms interpreted as early animals were present in the of the late . Many modern animal phyla became clearly established in the as during the , which began around 542 million years ago. 6,331 groups of s common to all living animals have been identified; these may have arisen from a single that lived . Phylogenetic tree |label2= |2= |label2= |sublabel2=680 mya |2= |2= }} |label2= |sublabel2=Triploblasts |2= |1= |2= |label3= |sublabel3=650 mya |3= |1= |1= |label2= |2= |2= / }} }} |label2= |2= |2= }} }} |label2 = |sublabel2=610 mya |2= |sublabel1=>529 mya |1= |2= and allies |2= and allies }} }} |label2= |2= |1= and allies |2= }} |label2= |sublabel2=580 mya |2= and allies |label2= |sublabel2=550 mya |2= and allies |2= and allies }} }} }} }} }} }} }} }} }} }} Glide reflection In 2-dimensional , a glide reflection (or transflection) is a that consists of a over a line and then along that line, combined into a single operation. Glide symmetry can be observed in nature among certain fossils of the ; the s; and certain worms. It can also be seen in many extant groups of s. Dickinsonia Dickinsonia is an extinct genus of fossils of the . The individual Dickinsonia typically resembles a bilaterally symmetrical ribbed oval. Its are presently unknown; its mode of growth is consistent with a n affinity, though some have suggested that it belongs to the , or even an "extinct ". The discovery of molecules in fossils of Dickinsonia lends support to the idea that Dickinsonia was an . Dickinsonia fossils are known only in the form of imprints and casts in sandstone beds. The specimens found range from a few s to about in length, and from a fraction of a millimetre to a few millimetres thick. They are nearly bilaterally symmetric, segmented, round or oval in outline, slightly expanded to one end (i.e. egg-shaped outline). The rib-like segments are radially inclined towards the wide and narrow ends, and the width and length of the segments increases towards the wide end of the fossil. The segments are separated by a thin ridge or groove along the axis of symmetry into right and left halves. The segments are organized in an alternating pattern according to symmetry rather than bilateral symmetry; thus, these " " are . This glide reflection is also found in , another mysterious organism from approximately the same era. The segments of Dickinsonia have been described by as " s", chambers filled with a liquid at higher than ambient pressure, analogous to a quilted . Some spectacular fossils attributed to Dickinsonia appear to preserve internal anatomy, believed to represent a tract that both digested food and distributed it throughout the organism. Charnia in }} Charnia is a of -like lifeforms with segmented, leaf-like ridges branching alternately to the right and left from a zig-zag medial suture (thus exhibiting , or opposite isometry). The genus Charnia was named after in , England, where the first fossilised specimen was found. Charnia is significant because it was the first fossil to be recognized as such. The living organism was a type of life form that grew on the sea floor and is believed to have fed on nutrients in the water. Despite Charnia's fern-like appearance, it is not a photosynthetic plant or alga because the nature of the fossilbeds where specimens have been found implies that it originally lived in deep water, well below the where can occur. Little is known about the of Charnia. It was and , anchored to the sea floor. According to one currently popular hypothesis, it probably lived in deep waters, well below the , thus placing it out of range of photosynthesis. Furthermore, it has no obvious feeding apparatus ( , , etc.) so its lifestyle remains enigmatic. Some have speculated that it survived either by or directly absorbing nutrients, and this is currently the focus of considerable research. The growth and development of the Ediacara biota is also a subject of continued research, and this has discredited the sea pen hypothesis. In contrast to , which grow by basal insertion, Charnia grew by the apical insertion of new buds. References Category:Tree of life